1. Field of the Invention
The present invention relates to a device and method for curing photosensitive compositions using an exposure device containing a light guide such as an optical fiber. More particularly, the invention pertains to a light guide exposure device for curing photosensitive compositions by ultraviolet (UV)/violet light generated by light emitting diodes and directed to the photosensitive composition by the light guide.
2. Description of the Related Art
Ultraviolet curable photosensitive compositions are used in the manufacture of electronic components, medical equipment, and other industrial products. The photocuring compositions found in such environments are typically cured using mercury-arc lamps to flood the UV sensitive material with UV light. While mercury-arc lamp technology is widely used, such technology has several disadvantages. One disadvantage is the relatively short life span of the mercury bulbs used in the mercury-arc lamps. Furthermore, the mercury-arc bulb degrades nonlinearly during its lifetime. As a result, conventional mercury-arc photocuring systems often require monitoring and adjusting the output power as the bulb degrades. Further, mercury-arc lamps are typically powered-on even during stand-by periods because they require cumbersome warm-up and cool-down cycles. As a result, much of the life of the mercury bulbs may be lost during these stand-by periods. Another disadvantage is the broad spectrum of the light radiated by the mercury-arc lamp. A mercury-arc lamp radiates UV, visible, and infrared (IR) light. Typically, UV band pass filters transmit the portion of the UV spectrum required for curing a particular photosensitive material. Heat-rejecting IR filters are usually employed to prevent heating of the cure surface. Because the IR radiation creates a very hot lamp housing, transmission optics near the lamp housing must be made of temperature resistant, UV-transmissive materials.
A wide variety of means are known to control the illumination of a light exposure apparatus. For example, the illumination from the light source can be controlled by changing the current or voltage of the light source or by changing the effective phase section of the current. U.S. Pat. No. 6,515,430 shows a typical light exposure device including a lamp for shining light through a light delivery device which can be an optical fiber. Between the lamp and the optical fiber are a shutter and a diaphragm which controls the duration and intensity of light sent from the lamp to the optical fiber.
In other illumination control techniques, a neutral or gray wedge, a filter or a similar absorptive device can be placed in the optical path between the light source and the optical fiber. However, varying the illumination density by means of a gray wedge is limited and does not permit unobstructed directing of light, but will always reduce maximum illumination. It is also known in the art that the brightness of the light emitted by optical lamps may be controlled by means of a built-in iris diaphragm. U.S. Pat. Nos. 2,735,929 and 3,222,510 show a standard iris diaphragm. However, an iris diaphragm will change the entrance angle of the light into the optical fiber and the exit angle of the light emanating from the optical fiber.
U.S. Pat. No. 7,273,369 shows an optical fiber light module which includes a hollow housing including at least one heat sink, and two or more light sources such as LED arrays. U.S. Pat. No. 4,948,214 shows a lens array for optical scanning devices including a light guide and microlens device for LED imaging. U.S. Pat. No. 6,260,994 shows an endoscope structure which includes LEDs arranged in a matrix at an end of a cylindrically-shaped housing, a light guide for guiding light from the LEDs forward, and a focusing lens. U.S. Pat. No. 6,645,230 shows a structure including an array of LEDs mounted on or within a housing. U.S. Pat. No. 7,218,830 shows a flat panel light guide containing several light-guide members and at least one point light source, such as an LED or array of LEDs. U.S. Pat. No. 5,204,224 relates to a method which includes exposing a photoresist on a wafer to light from an optical fiber light guide, subjecting the photoresist to a second exposure, and developing the photoresist to remove non-exposed areas. No LED or LED arrays are mentioned in this reference. U.S. Pat. No. 7,134,768 relates to an LED lamp with light guide, for use in vehicle lamps, traffic signal lamps, video games, and other lighting applications. The structure includes a plurality of LEDs whose light is collimated via a light guide, and inwardly reflected via multiple reflective surfaces within a housing. U.S. Pat. No. 7,194,185 relates to electronic devices having a colored light guide protruding through a cover. The light guide is illuminated by two LEDs below the cover. The two LEDs are in contact with a second light guide, below the cover, which is capable of blending the light colors from the two LEDs. U.S. Pat. No. 6,880,954 shows a method and apparatus for curing photosensitive materials using LEDs and an optical concentrator to generate high optical power intensities. This reference uses both collimated light LEDs and an optical element between the LEDs and an optical fiber. It has been determined that a problem with these aperture arrangements is non-uniformity of light intensity.
The introduction of UV/violet light emitting diodes (LEDs) has created new alternatives for curing some UV/violet sensitive materials. LED technology offers several advantages over the traditional mercury-arc technology. Typical LEDs last between 50,000 to 100,000 hours, providing a significant lifespan improvement over mercury-arc technologies. UV/violet LEDs also do not emit significant amounts of IR radiation, so heat-rejecting IR filtration is not required. As an added benefit, the reduced heat generation allows the use of economical UV/violet transmitting polymers for optics.
LED sources can also be turned on and off as required because LEDs do not require the warm-up and cool-down periods common in mercury-arc lamp systems. Some LED curing systems may implement driver circuits to control the current supplied to the LEDs. These circuits typically use a closed-loop system to monitor and control the output power of the LEDs, by controlling the drive current, to provide a stable and reliable UV/violet source. These circuits may also define different curing cycles for different photosensitive materials, such as emitting a specific output power for a specific length of time.
Unfortunately, conventional LED sources and LED systems have relatively low output power compared to traditional mercury-arc lamps. While the lower output power LED photocuring systems have proven to be sufficient for some dental applications, many commercial and industrial UV/violet sensitive materials require higher output powers to quickly cure the materials in a fast production environment.
The present invention provides an improved light guide exposure device having improved uniformity and amount of light intensity. A single light emitting diode array is positioned directly adjacent to a proximal end of a flexible light guide in substantial alignment with the light guide proximal end to direct substantially all of the light from the array to the proximal end. There are no optical elements between the light emitting diodes and the proximal end which would decrease the intensity of light directed from the light emitting diodes to the proximal end.